Crude oils contain heteroatomic polyaromatic molecules that include compounds such as sulfur, nitrogen, nickel, vanadium and others in quantities that can adversely effect the refinery processing of the crude oil fractions. Light crude oils or condensates have sulfur concentrations as low as 0.01 percent by weight (W %). In contrast, heavy crude oils and heavy petroleum fractions have sulfur concentrations as high as 5-6 W %. Similarly, the nitrogen content of crude oils can be in the range of 0.001-1.0 W %. These impurities must be removed during refining to meet established environmental regulations for the final products (e.g., gasoline, diesel, fuel oil), or for the intermediate refining streams that are to be processed for further upgrading, such as isomerization reforming. Contaminants such as nitrogen, sulfur and heavy metals are known to deactivate or poison catalysts.
Asphaltenes, sometime also referred to as asphalthenes, which are solid in nature and comprise polynuclear aromatics present in the solution of smaller aromatics and resin molecules, are also present in the crude oils and heavy fractions in varying quantities. Asphaltenes do not exist in all of the condensates or in light crude oils; however, they are present in relatively large quantities in heavy crude oils and petroleum fractions. Asphaltenes are insoluble components or fractions and their concentrations are defined as the amount of asphaltenes precipitated by addition of an n-paraffin solvent to the feedstock as prescribed in the Institute of Petroleum Method IP-143.
The chemical structure of asphaltenes are complex and are comprised of polynuclear hydrocarbons of molecular weight up to 20,000 joined by alkyl chains. Asphaltenes include nitrogen, sulfur and oxygen. Asphaltene has been defined as the component of a heavy crude oil fraction that is precipitated by addition of a low-boiling paraffin solvent, or paraffin naphtha, such as normal pentane, and is soluble in carbon disulfide and benzene. The heavy fraction can contain asphaltenes when it is derived from carbonaceous sources such as petroleum, coal or oil shale. Asphaltogenic compounds are present in petroleum in insignificant quantities. There is a close relationship between asphaltenes, resins and high molecular weight polycyclic hydrocarbons. Asphaltenes are hypothesized to be formed by the oxidation of natural resins. The hydrogenation of asphaltic compounds containing neutral resins and asphaltene produces heavy hydrocarbon oils, i.e., neutral resins and asphaltenes are hydrogenated into polycyclic aromatic or hydroaromatic hydrocarbons. They differ from polycyclic aromatic hydrocarbons by the presence of oxygen and sulfur in varied amounts.
Upon heating above 300°-400° C., asphaltenes are not melted, but decompose, forming carbon and volatile products. They react with sulfuric acid to form sulfonic acids, as might be expected on the basis of the polyaromatic structure of these components. Flocs and aggregates of asphaltene will result from the addition of non-polar solvents, e.g., paraffinic solvents, to crude oil and other heavy hydrocarbon oil feedstocks.
In a typical refinery, crude oil is first fractionated in the atmospheric distillation column to separate sour gas including methane, ethane, propanes, butanes and hydrogen sulfide, naphtha (36°-180° C.), kerosene (180°-240° C.), gas oil (240°-370° C.) and atmospheric residue, which are the hydrocarbon fractions boiling above 370° C. The atmospheric residue from the atmospheric distillation column is either used as fuel oil or sent to a vacuum distillation unit, depending upon the configuration of the refinery. Principal products from the vacuum distillation are vacuum gas oil, comprising hydrocarbons boiling in the range 370°-520° C., and vacuum residue, comprising hydrocarbons boiling above 520° C.
Naphtha, kerosene and gas oil streams derived from crude oils or other natural sources, such as shale oils, bitumens and tar sands, are treated to remove the contaminants, such as sulfur, that exceed the specification set for the end product(s). Hydrotreating is the most common refining technology used to remove these contaminants. Vacuum gas oil is processed in a hydrocracking unit to produce gasoline and diesel, or in a fluid catalytic cracking (FCC) unit to produce mainly gasoline, low cycle oil (LCO) and high cycle oil (HCO) as by-products, the former being used as a blending component in either the diesel pool or in fuel oil, the latter being sent directly to the fuel oil pool.
There are several processing options for the vacuum residue fraction, including hydroprocessing, coking, visbreaking, gasification and solvent deasphalting. Solvent deasphalting is practiced commercially worldwide. In the solvent deasphalting process, the asphalt fraction comprising 6-8 W % of hydrogen is separated from the vacuum residue by contact with a paraffinic solvent (carbon number ranging from 3-8) at elevated temperatures and pressures. The deasphalted oil comprising 9-11 W % hydrogen, is characterized as a heavy hydrocarbon fraction that is free of asphaltene molecules and can be sent to other conversion units such as a hydrocracking unit or a fluid catalytic cracking unit for further processing.
The deasphalted oil contains a high concentration of such contaminants as sulfur, nitrogen and Conradson which is an indicator of the coke forming properties of heavy hydrocarbons and defined as micro-Conradson residue (MCR) or Conradson carbon residue (CCR). MCR is determined by ASTM Method D-4530. In this test, the residue remaining after a specified period of evaporation and pyrolysis is expressed as a percentage of the original sample For example, deasphalted oil obtained from vacuum residue of an Arabian crude oil, contains 4.4 W % of sulfur, 2,700 ppmw of nitrogen and 11 W % of micro-carbon residue. In another example, a deasphalted oil of Far East origin contains 0.14 W % sulfur, 2,500 ppmw of nitrogen and 5.5 W % of CCR. These high levels of contaminants, and particularly nitrogen, in the deasphalted oil cause poor performance in conversion in hydrocracking or FCC units. The adverse effects of nitrogen and micro-carbon residue in FCC operations has been reported to be as follows: 0.4-0.6 higher coke yield, 4-6 V % less gasoline yield and 5-8 V % less conversion per 1000 ppmw of nitrogen. (See Sok Yui et al., Oil and Gas Journal, Jan. 19, 1998.) Similarly, coke yield is 0.33-0.6 W % more for each one W % of MCR in the feedstock. In hydrocracking operations, the catalyst deactivation is a function of the feedstock nitrogen and MCR content. The catalyst deactivation is about 3-5° C. per 1000 ppmw of nitrogen and 2-4° C. for each one W % of MCR.
It has been established that organic nitrogen is the most detrimental catalyst poison present in the hydrocarbon streams from the sources identified above. The organic nitrogen compounds poison the active catalytic sites which results in the deactivation of the catalyst, which in turn adversely effects the catalyst cycle or process length, the life of the catalyst, product yields, product quality, increases the severity of operating conditions and the associated cost of plant construction and operations. Removing nitrogen, sulfur, metals and other contaminants that poison catalysts will improve refining operations and will have the advantage of permitting refiners to process more and/or heavier feedstocks.
A number of processes have been disclosed for deasphalting of hydrocarbon oils that are based upon the use of paraffinic solvents that cause the asphaltenes to form a precipitate that can be recovered.
In U.S. Pat. No. 4,816,140, a process is described for deasphalting a hydrocarbon oil with a solvent having 3-8 carbon atoms, resulting in an asphaltic phase and a solution of deasphalted oil in the solvent. The solvent is then separated from the deasphalted oil, by passing the solution across an inorganic membrane of pore radii from 2 to 15 nanometers. The deasphalted oil is selectively retained on the upstream side of the membrane.
In U.S. Pat. No. 4,810,367, a process for deasphalting a heavy hydrocarbon feedstock is disclosed, comprising two stages of precipitation from the feedstock of an asphaltene fraction alone or, alternatively, of a resin fraction along with the asphaltene fraction, by means of a heavy solvent and a light solvent, respectively. In accordance with the process, the heavy solvent and the light solvent both contain, in different proportions, at least one hydrocarbon having 3 carbon atoms and at least one hydrocarbon having at least 5 carbon atoms, the proportion of the hydrocarbon having 3 carbon atoms being higher in the light solvent than in the heavy solvent.
In U.S. Pat. No. 4,747,936, a process for deasphalting and demetallizing heavy oils includes a counter-flow washing step which increases the yield of the product oil by contacting a heavy oil feedstream in countercurrent flow with a solvent in a multi-stage extraction zone and a resulting light phase stream is heated and passed into a settling zone. A second light phase stream comprised of the deasphalted product and demetallized oil and solvent is separated in the settling zone from a contaminant-laden heavy phase which is also termed a resin phase. The settling zone contains an equilibrium amount of DMO and solvent. DMO-enriched solvent is displaced from the resin stream by means of a counter-flow washing process using pure solvent.
In U.S. Pat. No. 4,572,781, a process for solvent deasphalting in solid phase is described that separates substantially dry asphaltenes of high softening point from heavy hydrocarbon material, comprising several steps described as: (a) admixing heavy hydrocarbon material containing asphaltenes with a solution of deasphalted oil and an aliphatic hydrocarbon precipitant in a first mixing zone to form a mixture and precipitate asphaltenes; (b) in a first separation zone the mixture from step (a) into (i) a first solution of deasphalted oil and precipitant and (ii) a slurry of solid asphaltene particles in a solution of precipitant and deasphalted oil; (c) separating the first solution of step (b) to obtain said precipitant and the deasphalted oil almost free of asphaltenes; (d) introducing the slurry of asphaltenes of step (b) into a second mixing zone and washing the slurry with a volume of fresh precipitant to remove deasphalted oil; (e) introducing the mixture from the second mixing zone into a second separation zone that comprises a centrifugal decanter to separate a liquid phase from a highly concentrated slurry of solid asphaltene; (f) recycling the liquid phase from the second separation zone to said first mixing zone; (g) introducing the concentrated slurry of solid asphaltenes from the second separation zone into a solvent removal system to recover the solvent and to obtain a product comprising fine particles of high softening point asphaltenes; and (h) recycling the solvent recovered in the solvent removal system to the second mixing zone.
In U.S. Pat. No. 4,502,944, a process for fractionation of heavy hydrocarbon process material resins and asphaltenes into at least three fractions is disclosed. The process material is mixed in a mixing zone with a solvent selected from the group consisting of paraffinic hydrocarbons having between about 3 to about 8 carbon atoms. The process material-solvent mixture is introduced into a first separation zone to form an asphaltenes-rich first heavy fraction and a resin-rich intermediate fraction, separated by a first liquid-liquid interface, and to form a first light fraction, rich in solvent and oils, separated from the intermediate fraction by a second liquid-liquid interface. The first heavy fraction and the intermediate fraction are withdrawn from the first separation zone. The first light fraction is introduced into a second separation zone to separate a second heavy fraction, rich in oils, and a second light fraction, rich in solvent.
In U.S. Pat. No. 4,411,790, a process for the treatment of a hydrocarbon charge by high temperature ultrafiltration is disclosed which is said to be useful for the regeneration of waste oil and to the reduction of the rate of asphaltenes in a hydrocarbon charge. The process comprises the steps of circulating the charge in a module having at least one mineral ultrafiltration barrier coated with a sensitive mineral layer of at least one metal oxide and of operating at a temperature higher than 100° C. The barrier, which preferably has a ceramic or metallic support, is coated with a sensitive layer selected from titanium dioxide, magnesium oxide, aluminum oxide, spinel MgAl2O4, and silica.
In U.S. Pat. No. 4,239,616, a process is described for effecting a deep cut in a heavy hydrocarbon material without a decrease in the quality of the extracted oil caused by the presence of undesirable entrained resinous bodies. The heavy hydrocarbon material is admixed with a solvent and introduced into a first separation zone maintained at an elevated temperature and pressure to effect a separation of the feed into a first light phase and a first heavy phase comprising asphaltenes and some solvent. The first light phase is introduced into a second separation zone maintained at an elevated temperature and pressure to effect a separation of the first light phase into a second light phase comprising oils and solvent and a second heavy phase comprising resins and some solvent. A portion of the first heavy phase is withdrawn and introduced into an upper portion of the second separation zone to contact the second light phase, after which it separates therefrom. This contact removes at least a portion of any entrained resinous bodies from the oil contained in the second light phase.
In U.S. Pat. No. 4,305,814, an energy efficient process for separating hydrocarbonaceous materials into various fractions is disclosed. The hydrocarbonaceous material is admixed with a solvent and the mixture is introduced into a first separation zone maintained at an elevated first temperature and pressure. The feed mixture separates into a first light phase comprising solvent and at least a portion of the lightest hydrocarbonaceous material and a first heavy phase comprising the remainder of the hydrocarbonaceous material and some solvent. The first heavy phase is introduced into a second separation zone maintained at a second temperature level above the first temperature level and at an elevated pressure. The first heavy phase separates into a second light phase comprising solvent and a second heavy phase comprising at least a portion of the hydrocarbonaceous material. The separated hydrocarbonaceous material fractions are recovered.
In U.S. Pat. No. 4,290,880, a supercritical process for producing deasphalted demetallized and deresined oils is disclosed. A process for effecting a deep cut in a heavy hydrocarbon material without a decrease in the quality of the extracted oil caused by the presence of undesirable entrained resinous bodies and organometallic compounds. The heavy hydrocarbon material is contacted with a solvent in a first separation zone maintained at an elevated temperature and pressure to effect a separation of the feed into a first light phase and a first heavy phase comprising asphaltenes and some solvent. The first light phase is introduced into a second separation zone maintained at an elevated temperature and pressure to effect a separation of the first light phase into a second light phase comprising oils and solvent and a second heavy phase comprising resins and some solvent. A portion of the second heavy phase is withdrawn and introduced into an upper portion of the second separation zone to counter-currently contact the second light phase. This contact removes at least a portion of any entrained resinous bodies and organometallic compounds from the oils contained in the second light phase.
A supercritical extraction process is disclosed in U.S. Pat. No. 4,482,453 in which the recovery of hydrocarbon values from a feedstream with high metals content can be carried out more efficiently via supercritical extraction with the recycle of a portion of the asphalt product and proper control of a countercurrent solvent flow during extraction.
In U.S. Pat. No. 4,663,028, a process of preparing a donor solvent for coal liquefaction is described in which liquefied coal is distilled to separate the coal into a fraction having a boiling point less than about 350° F. and a fraction having a boiling pit greater than about 350° F. The residue from the distillation is deasphalted in a first solvent capable of substantially extracting a first oil comprising lower molecular weight compounds and saturated compounds. The residue from the first deasphalting step is then deasphalted in a second solvent capable of substantially extracting a second oil comprising concentrated aromatic and heterocyclic compounds and leaving in the residue asphaltenes and ash. The second oil can be used as a donor solvent. The second oil extracted in the second deasphalting step is preferably partially hydrogenated prior to use as a donor solvent for the liquefaction of coal.
The prior art processes described above utilize various solvent extraction schemes for deasphalting petroleum fractions to improve the quality of the downstream products and the overall efficiency of the refinery. However, additional improvements in product quality and process efficiency are highly desirable.
It is therefore an object of the present invention to provide an improved solvent deasphalting process in which the treated feedstock will have a substantially reduced level of such contaminants as nitrogen, sulfur and metal compounds.
Another object of the invention is to provide an improved solvent deasphalting process in which the solvents are recovered and recycled for use.
It is also an object of the invention to provide an improved process for solvent deasphalting of a heavy residue oil or fraction that is efficient and effective under relatively mild and easily controlled conditions, thereby providing versatility.
The process is applicable to naturally occurring hydrocarbons such as crude oils, bitumens, heavy oils, shale oils and refinery streams that include atmospheric and vacuum residues, fluid catalytic cracking slurry oils, coker bottoms, visbreaking bottoms and coal liquefaction by-products.